In many cases, an electric power system comprises a direct voltage rail, one or more battery elements for supplying electric energy to the direct voltage rail, and one or more load-converters for converting the direct voltage of the direct voltage rail into voltages suitable for one or more loads of the electric power system. The electric power system can be for example an electric power system of a ship in which case the loads of the electric power system may comprise one or more propulsion motors, an alternating voltage network of the ship, and other loads such as e.g. one or more bow thruster motors. The motors are advantageously alternating current “AC” motors and the corresponding load-converters are inverters for converting the direct voltage of the direct voltage rail into alternating voltages suitable for the AC-motors.
In many cases it is advantageous that the direct voltage of the direct voltage rail is higher than the direct voltages of the battery elements. In these cases, each of the battery elements is typically connected with a voltage-increasing supply-converter to the direct voltage rail. The supply-converter comprises typically an inductor coil whose first pole is connected to the respective battery element, a controllable switch between the ground and the second pole of the inductor coil, and an unidirectionally conductive component, e.g. a diode, for providing a path for electric current from the inductor coil towards the direct voltage rail in response to a situation in which the controllable switch is in a non-conductive state.
In an electric power system of the kind described above, there is typically a need for a selective protection so that, in a case of a fault, a portion of the electric power system which is functionally separated from the rest of the electric power system is as small as possible. For example, in a case of fault in one of the load-converters, only the faulty load-converter is functionally separated from the rest of the electric power system. In order to implement the selective protection, each of the above-mentioned supply-converters is typically connected via a fuse or another over-current protector to the direct voltage rail. Correspondingly, each of the above-mentioned load-converters is connected via a fuse or another over-current protector to the direct voltage rail.
An inherent challenge related to the above-described approach is that, in many fault situations, the supply-converters of the kind described above may be incapable of supplying sufficient fault current within a sufficiently short time after the beginning of a fault situation. Therefore, the fault current through a fuse or another over-current protector may be insufficient to burn the fuse or to switch the other over-current protector into a non-conductive state sufficiently fast. Therefore, there is a risk that a faulty portion of the electric power system is not correctly separated from the rest of the electric power system. It is naturally possible to provide the supply-converters with additional means for supplying sufficient fault current from the battery elements within a sufficiently short time but this would make the supply-converters significantly more complex and less cost effective.
Publication WO2015168830 describes a method for protecting a direct-current “DC” electric power distribution system that includes one or more alternating current/direct current “AC/DC” converters and/or one more DC/DC converters, and one or more loads, connected by DC-buses. The method, which is carried out in response to a detection of a fault somewhere in the system, begins with limiting an output current of each of one or more of the converters so that each of these converters outputs a limited DC-current. After the current limitation of the one or more converters has taken place, one or more protection devices in the system are activated, where the activating at least partly depends on the limited DC-currents. The activation may comprise an automatic opening of one or more protection devices, wherein the opening of each protection device is based on a respective device current exceeding a respective threshold for a respective period of time. In this method, a correct operation of the protection devices is achieved so that the limited DC-currents are controlled so that the activation of the protection devices is successful. This approach, however, complicates the control of the converters.